Ponyo is a sweet-faced goldfish from a magical realm under the sea. One day, Ponyo's curiosity takes her far from home. After accidentally hitching a ride on a jellyfish, Ponyo washes up on the shore and is found by Sosuke, a kindly human boy. The two become fast friends and are swept away on an incredible adventure.

After discovering Ponyo trapped in a tidepool, Sosuke takes her home and does his best to make sure she's happy and well fed in the human world. Ponyo's father, a powerful underwater sorcerer, sends out mysterious tidal forces to locate his daughter. Ponyo and Sosuke may be torn apart just when they've become best friends!